Much of the job growth in America today is in the service sector. The thing about service jobs though, is that they often don’t pay much.

But, there are a few exceptions to that rule, and this story is about one of those exceptions. Namely, the unionized jobs that keep the San Francisco Bay Area Rapid Transit system, known as BART, running.

As you’ve probably heard, BART workers are in the midst of a contract dispute right now, over pay and benefits. BART workers went on strike once this summer, snarling traffic and enraging commuters across the Bay Area. Employees and management are in the midst of a 60-day cooling-off period, but with little progress reported so far, another strike could be coming.

But with all the talk of how much BART workers should be getting, there's little discussion about what they actually do. To find out, I spent some time with one BART worker recently -- a “System Service Worker,” named Sharina Pearson.

"System Service Worker" is the technical name for what is basically a BART station janitor. Pearson is assigned to clean two stations, in an eight-hour shift that starts at 6 a.m. When I visit her, she is at the BART station in Fremont, a suburb in the East Bay, downstairs in the men's room. It is her fourth bathroom round of the day, and she is replenishing toilet paper and flushing unflushed toilets.

Next, she is up on the train platforms to sweep, scrape pigeon poop off benches, empty garbage cans and dodge passengers while she picks up trash where it eddies at the top of the escalators.And then, she does all that all over again.

"A lot of what I do," Pearson says, "is repetition."

There is lots of bending, lifting, pushing, pulling. And walking, briskly. At one point, she politely asks if I could pick up the pace.

This job earns Pearson about $52,000 a year, plus health care benefits, to which she contributes $92 each month. Though some BART workers accrue lots of overtime on top of this, she tells me overtime is rare with a janitorial position like hers.

Her salary, she says, affords a modest but decent life for her and her four kids, including an $1,800 monthly mortgage payment on a three-bedroom home in a Bay Area suburb where her commute is long, but housing is cheaper.

Pearson says after utilities, car payments and car insurance, food and clothes for her family, she often lives “paycheck to paycheck.” But she says she thanks God for finding what is a rare thing in America these days: A middle class job that does not require a college degree.

“It was nothing but a higher power” that helped her land the job at BART six years ago, she says.

And yet, Pearson is well aware of the critics who think her job is pretty cushy. Sometimes, one of those critics is her ten-year-old son, Malachai, who I met when Pearson was done with her shift.

Malachai says he once visited his mom on "Take Your Child to Work Day", and he wasn’t too impressed.

"I saw her clean the bathrooms," he says. "It looked kinda easy."

His mom laughed and told him: "I probably made it look easy."

Malachai thinks about it for a moment and adds that when he tried helping her, it was actually harder than it looked.

But Pearson would be the first to agree -- her job isn't intellectually challenging or stimulating in the same way as those of the lawyers or software developers who pass her by on their BART commutes.

Still, she says her job takes focus and problem-solving. And a strong stomach -- that becomes clear back on the BART train platform, when Pearson points down to show me something.

"That's where the hot lunch was, right there," she says.

A "hot lunch," I learn, is BART-worker lingo for a bodily fluid that needs immediate clean up. It's pretty common, and Pearson had had to deal with one as soon as she got to work the day I met her. A passenger had thrown up on the platform. When I heard this, I inadvertently made a face.

"It is what it is," she says. "That's what I get paid to do."

It’s not what she wants to do always though: She sees this job as just one step in a journey.

She’s a few classes away from an associate's degree in business administration, and once her kids are a little older — her oldest starts college this fall, Pearson says she plans to go back to school herself.

And then there's her passion -- songwriting.

There, by the escalator, in her blue BART janitor’s uniform, she breaks in to song.

Let me tell you about a friend of mine

Says she wants to be happy…

She sings, in a clear, velvety voice, while the passengers walk by doing double-takes.

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Krissy Clark is the senior correspondent for Marketplace’s Wealth & Poverty Desk, where she helps make sense of some of the most fundamental shifts happening in the U.S. economy, including the growth of the low-wage service sector and the shrinking of middle-wage, middle-class...